The invention relates to a method for measuring a frequency datum, in particular a rotation speed datum in the context of a motor, and to an apparatus for carrying out such a method.
Digital systems for sensing the rotation speed of a rotating object (xe2x80x9crotorxe2x80x9d) relate to the fact that such a rotor generates one or more signals with each revolution, e.g. needle pulses, pulse edges, or the like. These signals will be referred to hereinafter as xe2x80x9cevents.xe2x80x9d The sensing of a rotation speed datum is based on the fact that the time interval between xe2x80x9ceventsxe2x80x9d of this kind is measured, or that a count is made of the number of such xe2x80x9ceventsxe2x80x9d occurring per unit time.
Since the number of such xe2x80x9ceventsxe2x80x9d per unit time increases with increasing rotation speed, correspondingly more calculation time is required to process them in a digital system as the rotation speed rises, so that the calculation time available for other processes decreases.
It is an object of the invention to make available a new method for measuring a frequency datum, in particular a rotation speed datum in the context of a motor, and an apparatus for carrying out such a method.
According to a first aspect of the invention, this object is achieved by calculating interval length over a period of several events or instants. The result thereof is that the measurement of the rotation speed datum begins approximately at the first predefined instant, and an approximate measurement duration is defined by the first predefined instant and the third predefined instant. This approximate measurement duration is not influenced by the rotational velocity of the rotor. The initiation of the calculation of the rotation speed datum, which calculation can be accomplished after the measurement is complete, is thus also approximately determined in time. As a result, a microprocessor or microcontroller (hereinafter xe2x80x9cxcexcCxe2x80x9d) that, for example, is present is under a uniform load at all rotation speeds, so that sufficient calculation performance is always available for other tasks as well, e.g. controlling the rotation speed.
Because, in the course of a measurement, the xe2x80x9ceventsxe2x80x9d are sensed during one complete rotor rotation or a plurality of complete rotor rotations, an averaging occurs during measurement of the rotation speed datum. Specifically, if the rotation speed is ascertained by way of a plurality of short measurements, e.g. by measuring during part of a revolution, it is then usually necessary to create a moving average by calculation from multiple measured values. With the invention, on the other hand, an average is obtained without additional calculation, thereby additionally relieving the load on, for example, a xcexcC. And despite the approximate definition of a measurement duration by way of the defined instants, with the method the actual measurement takes place precisely between two events of the sensor signal which are associated with the same rotational position of the rotor, i.e. the rotation speed datum is measured during complete revolutions. This is of interest particularly in the case of rotors which have, in order to generate the sensor at signal, multiple marks at differing angular distances. Thus in the case of a defined measurement, measurement occurs at the same mark, i.e. at the same rotational position of the rotor, so that any differences in angular distance play no part. This results in a highly accurate rotation speed measurement.
Another advantageous embodiment of the invention is when the 3rd instant of one measurement is also the 1st instant of a subsequent measurement. A method of this kind continuously measures a rotation speed datum, since the subsequent measurement follows the instantaneous measurement without interruption. In the vicinity of each defined instant, an instantaneous measurement is terminated and a new measurement is begun. If the predefined instants each occur after a time interval T13 A, then on average a measurement takes place in each case after the time T13 A. Processing of the measurement, for example in a xcexcC, can thus take place at regular intervals that are independent of the rotation speed. As a result, the load on the xcexcC is uniform at all rotation speeds, so that sufficient calculation time is left over for other tasks, e.g. for commutating an electronically commutated motor or for rotation speed control.
Since in this case there is no pause between the individual measurements, this method is particularly suitable if each individual revolution of the rotor must be taken into account for the measurement, as is necessary, for example, with motors having high-precision controllers or with stepping motors.
The invention further concerns a method in which the remainder from a first measurement calculation is taken into account when making a subsequent measurement calculation. The result of taking into account, according to the present invention, the remainder in the subsequent measurement is, when the rotation speed is calculated by division, that no rotation speed information is lost by rounding. This results very advantageously in an increase in measurement accuracy, and is highly advantageous in the context of complicated control operations which demand high precision.
The refinement at which successive measurements, made at the same rotor position, are compared has proven extraordinarily advantageous in this context, since a method of this kind is particularly tolerant of interference pulses. An interference pulse does cause an error in the instantaneous measurement, but the subsequent measurements are once again correct, although they use a different rotor position than before to sense the measurement duration.
An apparatus which includes a programmed-controlled device for processing the signals and making the time interval calculations also allows the aforesaid object to be achieved.
The preferred use of a ring counter has the advantage that the stopping point of the one measurement and the starting point of the next measurement are simultaneous because the ring counter is always running, so that no errors can occur due to measurement delay, and any delays resulting, for example, from the simultaneous occurrence of two interrupts are subsequently automatically compensated for. This makes possible both uninterrupted sensing of the rotation speed and an average error in the rotation speed sensing equal to zero, since no bits are lost during the measurement.
Further details and advantageous developments of the invention are evident from the exemplary embodiments described below and depicted in the drawings, which are in no way to be understood as a limitation of the invention.